The present invention pertains to automatic chroma level (saturation) control circuitry for a color television receiver and, more particularly, to a circuit for automatically controlling chroma gain to stabilize chroma level about a viewer-selected chroma level.
Incorrect color level can result in a television receiver due to errors in generation of the video in the studio and from transmission, and reception errors. Color level errors can cause color in the television display to appear either washed out or oversaturated. These errors may be particularly apparent between different channels or as a result of shifts from one type of program material to another (such as occurs during commercial interruptions) or as a result of shifts from one video camera to another at the studio.
A conventional type of automatic chroma control (ACC) system for television senses the magnitude of the received color burst signal and varies chroma gain to maintain a substantially constant burst level. This system assures a stable color level so long as the correct relationship is maintained between the burst signal and the chroma portion of the video signal and so long as system errors affect the burst and chroma signal equally. However, when the burst to chroma signal ratio is not maintained or if system errors occur which affect the burst and chroma signals unequally, operation of the automatic chroma control circuits may distort chroma saturation levels and noticeably degrade picture quality.
Errors in chroma level in the receiver, due either to generation, transmission or reception errors, or due to overcompensation by the ACC have been dealt with in the past by control circuits, commonly referred to as chroma averaging circuits. Such chroma averaging circuits are responsive to chroma amplitude changes in general to adjust chroma gain. In particular, these circuits are not selective of a particular range of hues and are not sensitive to particular brightness levels in the scene. Because of this lack of hue selectivity, such chroma averaging circuits can produce undersirably washed out pictures. For example, in the case of a high level green background as in a golf scene, the circuits will respond to reduce the overall chroma level so that any flesh tones within the scene are improperly reduced in level. This is particularly annoying since fleshtones, the known quantity to the viewer, are the predominant hues by which the viewer judges the quality of the color picture. Likewise, when chroma averaging circuits are not sensitive to the brightness level of the scene, in the presence of low brightness scenes where the chroma level is desirably low, such circuits will cause the chroma level to rise undesirably.